The invention relates generally to electronic measuring devices for measuring the propagation delay of an electrical, electromagnetic or acoustic signal between a transmitter and a receiver located at any distance from the transmitter, and more specifically to such a device where the signals delivered by the transmitter are square-wave pulses, and the signals reaching the receiver are groups of waves which increase in amplitude over several cycles and then decay, due to distortion of the pulses during the propagation, and whose period of vibration T is an integral multiple of the fundamental frequency of the transmitted pulses.
Measurements of the propagation delay between a transmitted and a received pulse signal can be made without an appreciable timing error only when the leading edge of the transmitted pulse arrives at the receiver at a definable instant. A prerequisite is that the pulse substantially retain its original shape during its propagation time so that the leading edge reaches the receiver with adequate steepness.
Frequently, however, this is not the case. For various reasons, either the transmitted pulse energy cannot be made large enough to exceed the trigger threshold set at the receiver, which must be well above the noise; or there are specific circumstances, as in the case of an ultrasonic measuring path for determining the flow rate of a medium, where the combined ultrasonic transmitter and receiver must transmit and receive the ultrasonic pulses through the wall of a tank. Since the tank may hold caustic media, such as acids or lyes, the tank is frequently made of high-grade steel several millimeters thick. In this case, the tank encloses the measuring path. The thickness of the wall of the tank and the length of the ultrasonic waves then are of about the same order of magnitude. A steel wall of this thickness will only be transparent to ultrasonic waves if its thickness is an integral multiple of one-half the ultrasonic wavelength in steel. However, the transmitted ultrasonic pulse, originally a square wave, will become so distorted in the transmission process that it arrives at the receiver, not as a pulse but as a group of waves that increases in amplitude over several cycles and then decays, and whose period of vibration is an integral multiple of the fundamental frequency of the transmitted pulse. The first appearance this wave train, which must be determined to measure the propagation time delay of the signal, gets obscured in the noise due to the amplitude and the small slope of the signal and the next vibration, and thus cannot be detected directly. Frequently, the voltage of the transmitted pulse cannot be simply increased in order to boost the received signal since in the case of many media the transmitting energy has to be kept low for safety reasons, e.g. in the petrochemical industry for explosion protection.
The present invention is directed to the problem of developing a method and device for determining the exact instant of the first appearance of a transmitted signal in applications where the transmitted pulse of an electrical, electromagnetic or acoustic nature arrives at the receiver as a group of waves that increases in amplitude and then decays, even when that instant is not distinguishable from the noise or then only with such small amplitude that the threshold is not measurable.